1. Field of the Invention
The present invention relates to an attachment structure of a vacuum pump for causing a negative pressure in an engine case in an engine, particularly in a four-cycle engine, of a vehicle such as a motorcycle.
2. Description of the Related Art
Conventionally, a four-cycle engine to be mounted in, for example, a motorcycle or the like is designed such that reciprocations of pistons to move in cylinder blocks are converted to rotation of a crankshaft housed in an engine case and motive power by the rotation of the crankshaft is output to the outside. In such a four-cycle engine, due to an increase in the number of rotations of the engine, various losses in terms of a machine, (which are referred to as what is called mechanical losses), are increased, and as one of the losses, a pumping loss is known. Here, the pumping loss means pressure resistance to the reciprocations of the pistons and is mainly ascribable to an air containing a blow-by gas in the engine case.
In order to reduce such a pumping loss, conventionally various devices have been made in an engine as has been disclosed in Patent Document 1, for example. That is, the engine disclosed in Patent Document 1 includes a vacuum pump to operate in conjunction with rotation of a crankshaft; and an air chamber communicating with an engine case, in which a negative pressure is caused in the air chamber by the vacuum pump. By the vacuum pump to operate in accordance with the number of rotations of the crankshaft, an air in the engine case is sucked and thus a negative pressure is always maintained in the engine case.
Further, an engine disclosed in Patent Document 2 is further designed such that an air mixed with gas and liquid in an engine case in which a crankshaft is housed is subjected to gas/liquid separation in a breather chamber provided upstream or downstream of a pump, and the discharge side of the pump is connected to intake and exhaust paths of a combustion chamber.
[Patent Document 1] Japanese Laid-open Patent Publication No. 05-60000
[Patent Document 2] Japanese Laid-open Patent Publication No, 2007-120411
However, when the breather chamber is provided between the engine and the pump, a task of pressurizing a gas having a small content of oil to discharge the pressurized gas is required for the pump. That is, the pump is required to have durability sufficient to endure self-heating in an adiabatic compression process and to maintain high airtightness even through a temperature environment is changed. Further, when the breather chamber is provided between the pump and the intake path or the exhaust path, the oil content passing through the pump chamber acts effectively for sealing performance and cooling performance, but a process of compressing the oil content causes a large mechanical loss, and thus if the breather chamber is provided as described above, an effect of reducing the pumping loss is canceled.
Particularly, in a V-type engine, a cylinder head, an intake pipe, and an exhaust pipe are disposed and constituted at an upper portion of an engine case. It is often difficult to dispose the breather and pump each having a sufficient capacity in such a restricted space.